An API, or Application Programming Interface, is a set of protocols, tools, and definitions that allow software applications to communicate with each other. It defines the methods and data formats that developers can use when interacting with external software components, such as operating systems, libraries, or services. APIs enable different systems to work together, making it easier to integrate various software applications and automate tasks.

APIs play a critical role in modern software development by enabling the creation of complex systems that leverage existing technologies and services. For example, a weather application might use an API to retrieve real-time data from a remote weather service, or an e-commerce platform might integrate with a payment gateway API to process transactions. By providing a standardized way to access functionality, APIs reduce the need to write custom code and promote modular, scalable software design.